With the recent increase in awareness of global environmental protection, systems with a power supply, such as a storage battery, a solar cell, and a fuel cell, have been developed. Such systems require a power supply unit so that DC power is converted into AC power and supplied to a load and commercial power supply. Further, in order to charge the storage battery from the commercial power supply, it is necessary to supply power to the storage battery by converting the AC power of the commercial power supply into DC power.
Patent document 1 (Japanese Unexamined Patent Application Publication No. 2002-78350) discloses an AC/DC bidirectional converter of a single-phase three-wire system. This converter allows for both charging from a single-phase three-wire AC power supply to a battery, and electric power regeneration of the single-phase three-wire AC power supply by the discharge of the battery by a single circuit, thereby achieving downsizing and low cost.
Patent document 2 (Japanese Unexamined Patent Application Publication No. 1997-140157) discloses an inverter unit using a solar cell that can switch between operation of interconnection with the commercial power system and self-sustained operation by a single inverter.
However, in the conventional single-phase three-wire AC/DC bidirectional converter described in Patent document 1, when the battery is charged from AC 100 V, the input current passes through two semiconductor devices such as a switching element and a diode. Thus, the conduction loss increases and it is difficult to achieve high efficiency. In addition, in the power regeneration of the single-phase three-wire AC power supply by the discharge of the battery, four switching elements are turned on and off. Thus, the switching loss increases and it is difficult to achieve high efficiency.
Further, in the conventional inverter unit using a solar cell described in Patent document 2, there is no disclosure of the technology of power supply both to the load of AC 100 V and the load of AC 200 V at the same time during a self-sustained operation.
Accordingly, an object of the present invention is to provide a highly efficient power supply unit compatible with the single-phase three-wire system, and an operation method of such a power supply unit.